PGPKey is alive again! - #80
 - Filled out EmbeddedSignature finally
 - Added EmbeddedSignatureHeader since Embeded Signatures don't have a tag or a length, just a version
 - Fixed a logic error in SubPackets.__contains__
 - Added tag class bases to *Key classes
 - Added a PacketTag enum
 - Shortened the names of the certification signature types in SignatureType
 - Expanded PGPSignature.prefs into separate methods
 - Added unit tests for PGPKey (for an rsa public key and an rsa private key)
 - This also brings us one step away from completing #76 and #71